1. Field of the Invention
The invention relates to an oriented multilayer polyolefin film comprising a base layer containing polypropylene, and at least one heat-sealable outer layer. The films are distinguished by a low coefficient of friction in combination with excellent heat-sealability and excellent processing properties.
2. Description of Related Art
The art describes various films having a low coefficient of friction. However, the demands made of the processing properties of films and their smooth passage through automatic machines have increased constantly over the years. For this reason, ever-lower coefficients of friction are demanded; today the term "low" friction values means in the order of from 0.3 to 0.1, while a few years ago, a friction value of from 0.4 to 0.5 was regarded as extremely low.
It is known from the art that the surface-slip characteristics of polyolefin films can be improved by adding a carboxamide. The films described contain amides in the outer layers or in the base and outer layers.
DE-A-2,001,032 describes films made from various thermoplastics whose surface-slip characteristics have been improved by addition of carboxamides and antiblocking agents. Since sufficient amounts of lubricants cannot be incorporated into the outer layers alone, the additional incorporation of the amides into the base layer is recommended. These films have a coefficient of friction in the range from 0.4 to 0.8 and thus no longer meet today's quality requirements.
U.S. Pat. No. 4,117,193 describes multilayer films comprising a polypropylene base layer containing a lubricant, an antiblocking agent and an antistatic. The outer layer of these films comprises a polymer blend, and additionally contains a lubricant and an antiblocking agent. The polymer blend comprises an ethylene-butylene copolymer and a polyolefinic resin such as HDPE or LDPE. It is stated that the deficient surface-slip characteristics of the films cannot be sufficiently improved by the addition of lubricants and antiblocking agents alone. For this reason, the outer layer is modified by addition of HDPE or LDPE in combination with a lubricant and antiblocking agent. According to the examples and comparative examples, the reduction in the coefficient of friction is essentially due to the addition of HDPE. Pure copolymer outer layers having the same additive composition have coefficients of friction of from 0.7 to 0.8. The films combine excellent coefficients of friction with good printability, but the addition of the friction-reducing polyolefinic resin means that they have very unsatisfactory haze and gloss.
The art furthermore states that an excellent lubricant for achieving friction values of less than 0.3 is the addition of silicone oil to polyolefinic films. Some publications recommend the use of the silicone oil in combination with other lubricants.
EP-A-0,182,463 describes a multilayer film containing from 0.05 to 0.2% by weight of tertiary aliphatic amine in the base layer and a combination of silicone oil and SiO.sub.2 in the heat-sealable outer layer. According to the description, the surprising interaction of SiO.sub.2, silicone oil and amine in combination with a selected outer layer thickness of less than 0.8 .mu.m gives films having a coefficient of friction of 0.3 or less. In spite of this excellent coefficient of friction, the processing properties of the film are deficient. In particular, it is not printable and is therefore unsuitable for many applications.
EP-A-0,143,130 discloses films containing a carboxamide in the base layer and likewise the combination of silicone oil and SiO.sub.2 in the outer layer. As in EP-A-0,182,463 mentioned above, a synergistic effect of the three selected components on the coefficient of friction is described. These films likewise have deficient processing properties, in spite of their advantageous surface slip. They too lack the important property of printability.
EP-A-0,194,588 and EP-A-0,217,598 describe a film which, in spite of good surface- slip characteristics, is readily printable in spite of the addition of silicone oil. In these films, silicone oil, if desired in combination with SiO.sub.2, is only incorporated into one outer layer. The second, silicone oil-free outer layer is corona-treated to improve the printability. The transfer of silicone oil onto the surface of this treated second outer layer subsequently takes place by contact with the first, silicone oil-containing outer layer. This trick gives a silicone oil-treated film having good surface-slip characteristics which is simultaneously readily printable on the corona-treated side, but is nevertheless heat-sealable.
This film has the disadvantage that it is only printable on one side. This is particularly disadvantageous for use of the film in the packaging sector. Packaging frequently requires further labeling on its second surface, for example a bar code, and for this reason requires a film having good surface-slip characteristics which is printable on both sides. It has furthermore proven disadvantageous that the pretreatment intensity of the corona-treated surface is no longer measureable due to the siloxane. This means that it is difficult for the film processor to check the surface tension of the film, an important quality feature. The processing properties of the film are also unsatisfactory.